Decreased radical recombination efficiency effects

FIGURE 7. The proposed effect of stress in the several stages of the decreased radical recombination efficiency hypothesis. 

It was proposed that the temperature dependence of polymer 5 arises from the temperature dependence of the kA step. Specifically, it was suggested that the polymer segments to which the radicals are attached are conformationally stressed. There are two possible modes for the newly formed radicals to relax and become separated They can rotate or recoil away from each other (Scheme 9). These secondary motions of the polymer arise from the relaxation of unfavorable bond conformations that are formed during the polymer casting process. The increased thermal energy facilitates the rotation and recoil relaxation processes, which effectively increases the rate constant for diffusion of the radicals out of the cage, kA. This leads to decreased radical-radical recombination and consequently an increase in photodegradation efficiency. 

The key feature of Inisurfs is their surfactant behavior. They form micelles and are adsorbed at interfaces, and as such they are characterized by a critical micelle concentration (CMC) and an area/molecule in the adsorbed state. This influences both the decomposition behavior and the radical efficiency, which are much lower than those for conventional, low molecular weight initiators. Tauer and Kosmella [4] have observed that in the emulsion polymerization of styrene, using an Inisurf concentration above the CMC resulted in an increase in the rate constant of the production of free radicals. This was attributed to micellar catalysis effects as described, for example, by Rieger [5]. Conversely, if the Inisurf concentration was below the CMC the rate constant of the production of free radicals decreased with an increase in the Inisurf concentration, which was attributed to enhanced radical recombination. Also note that a similar effect of the dependence of initiator efficiency on concentration was reported by Van Hook and Tobolsky for azobisisobutyronitrile (AIBN) [6]. 

A distribution of oxidized bases similar to that obtained with y-rays is observed upon exposure of human cells to high LET particles such as heavy ions [11]. However, with both and Ar + ions beams, a decrease in the yield of all lesions was observed [12]. This observation is reminiscent of the effect of increasing LET on the radiolytic yield of OH that decreases as the result of efficient radical recombination in the particle track. A same trend has been observed for both single and double-strand breaks [13]. These results strongly suggest a major role ofthe hydroxyl radical chemistry in the formation of radiation-induced... 

Recently we observed the effect which supports the conclusion about the substantial role of the radical reaction outside of the catalyst grains. When a very efficient OCM oxide catalyst (10% Nd/MgO) was placed into the reactor together with an inactive metal filament (Ni-based alloy) the sharp increase of conversion accompanied by the selectivity shift from oxidative coupling to the formation of CO and H2 was observed [19]. Since the metal component has a low activity also with respect to ethane oxidation, this behavior is not due to successive oxidation or decomposition of C2 hydrocarbons on the metal surface, but should be attributed to the reactions of methane oxidation intermediates. Almost total disappearance of ethane (which is a product of CH3 radicals recombination) and acceleration of the apparent reaction rate by the addition of an "inert material indicate that the efficiency of methane oxidative transformations can be substantially increased if the radicals have a chance to react outside the zone where they formed and the role of reaction (-1) decreases. Although the second (metal) surface is not active enough to conduct the reaction of saturated hydrocarbon molecules (methane and ethane), the radicals generated by the oxide can react further on the metal surface. As a result, the fraction of the products formed from methane activated in the reaction (1) increases, and the formation of the final reaction mixture of different composition takes place. 

The relative efficiencies of different benzoin derivatives vary considerably in photoinitia-tion. The nature of the monomer and the environment exert an effect as well. Also, the efficiency decreases when radical recombination is favored by the cage effect as a result of an increase in the viscosity of the medium. It is also possible that hydrogen abstractions in a viscous medium contribute to photoinitiations by benzoin and its derivatives. 

Increasing the temperature may favour the HCl diffusion out of the sample and decrease the autocatalytic effects [2182]. The initiation, propagation and termination steps in the thermal degradation of poly(vinyl chloride) at higher temperatures (190-220°C) are catalysed by HCl [121,954]. The low initiation efficiency results partly from the restricted mobility of the free radical fragments in a solid polymer irradiated at temperatures below (87°C), which favours radical recombination. By contrast, when poly(vinyl chloride)... 

The effect of various additives on a styrene polymerization reinforces the tentative conclusion that vinyl polymerization is not taking place. Even though yields were increased by halogenated additives they were not decreased by additives expected to act as scavengers for free-radical species (benzophenone) or ionic species (butylamine or water). Under an assumed mechanism of fragmentation and rapid recombination to condensed products, halogenated compounds additives may serve to increase the efficiency of energy transfer from the electric field to the monomer. 

This example demonstrates that free-radical polymerization could be the preferred mechanism for many vinyl monomers since, unlike ionic polymerization, it is tolerant of trace impurities and monomer functionality. However, one of its major drawbacks is the lack of control over the molecular weight distribution due to intrinsic termination reactions. Moreover, the efficiency factor of the initiator decreases by the so-called cage effect, for example by recombination of the primary fi-ee radicals, with increasing molecular weight of the macroinitiator [28]. This normally prevents the synthesis of block copolymers with controlled architectures, narrow molecular weight distributions and well-defined molecular weights. 

During radical pol3nnerization, the initiation rate depends on the way the initiator bonds with the surface. Chemisorption of initiator on the solid surface decreases the initiator decomposition rate, probably due to diminishing of the degree of freedom of initiator molecules which hinders diffusion separation of the radical pair. The restriction of mobility and recombination of radicals bound to the surface lowers the rate and effectiveness of pol3nnerization. The rate of thermal decomposition of initiators and their efficiency of initiation also depend... 

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